In a display device such as a liquid crystal display device or an organic EL display device, a pressure-sensitive adhesive is used to bond a transparent optical element, such as a polarizing film, a retardation film or a transparent cover element including a cover glass, to another optical element. That is, a pressure-sensitive adhesive layer is disposed between two optical elements to be bonded together, and then the two optical elements are pressed against each other and bonded together to thereby form an optical element laminate. In a display device, the optical element laminate configured as above is disposed such that the transparent optical element is located on a viewing side. This configuration involves a problem that, when outside light enters from the transparent optical element on the viewing side, the entered light is reflected at an interface between the pressure-sensitive adhesive layer and the optical element on a non-viewing side, and returned to the viewing side. This problem becomes prominent particularly when an entry angle of outside light is relatively large.
On the other hand, in a touch panel-equipped display device which has been increasingly becoming popular in late years, a transparent electroconductive layer such as a patterned ITO (Indium Tin Oxide) layer is formed on a surface of a bonding-target optical element to which a transparent optical element is to be bonded. In this type of display device, the problem “poor pattern invisibility” is pointed out that a pattern of the transparent electroconductive layer becomes visible from the viewing side, by the influence of internal reflection of entered light at an interface between a pressure-sensitive adhesive layer and the transparent electroconductive layer.
In each case, the internal reflection is caused by a difference in refractive index between the pressure-sensitive adhesive layer and each of the bonding-target optical element and the transparent electroconductive layer. JP 4640740 B (Patent Document 1) teaches a technique for addressing this problem. Specifically, the Patent Document 1 discloses a pressure-sensitive adhesive composition capable of reducing total reflection of light at an interface between a transparent optical element and a pressure-sensitive adhesive layer and at an interface between the pressure-sensitive adhesive layer and a bonding-target optical element. The composition disclosed in the Patent Document 1 is allegedly described as having a high refractive index in its dried and/or cured states, wherein it is close to refractive indexes of the transparent optical element and the bonding-target optical element body. The teaching of the Patent Document 1 is that a pressure-sensitive adhesive layer for bonding two optical elements together is entirely formed to have a refractive index close to refractive indexes of the two optical elements.
The technique taught by the Patent Document 1 would be effective in suppressing the interface reflection. On the other hand, there is a problem that the composition itself becomes costly, because this technique is based on the use of a particular monomer component.
JP 5564748 B (Patent Document 2) discloses a refractive index-adjusted pressure-sensitive adhesive which comprises a transparent base pressure-sensitive adhesive material comprised of an acrylic-based resin, and zirconium oxide or titanium oxide particles having an average dispersed particle size of 1 nm to 20 nm, wherein the zirconium oxide or titanium oxide particles are dispersed over the entire thickness of the transparent base pressure-sensitive adhesive layer. In this pressure-sensitive adhesive, the zirconium oxide or titanium oxide particles as a high refractive index material are mixed with the transparent base pressure-sensitive adhesive material. Thus, it is considered that a refractive index of a layer of the pressure-sensitive adhesive can be increased in its entirety to thereby suppress the aforementioned interface reflection. However, the technique disclosed in the Patent Document 2 requires using the high refractive index material in large amounts. This causes a concern about deterioration in properties required as a pressure-sensitive adhesive, and a problem of an increase in cost. Moreover, the high refractive index material used in the Patent Document 2 is particles of an inorganic material. Thus, there is another problem that the particles are hardly dispersed, causing whitish haze due to light scattering. In this regard, it is conceivable to use particles of an organic material. In this case, however, it becomes difficult to solve a problem of coloration.
With a view to improving the technique disclosed in the Patent Document 2, JP 5520752 B (Patent Document 3) proposes coating, with a polymer, metal oxide particles to be dispersed in a pressure-sensitive adhesive. The teaching of the Patent Document 3 is that, although the pressure-sensitive adhesive layer in the Patent Document 2 has a problem of deterioration in adherence property thereof because the metal oxide particles are exposed to a surface of the pressure-sensitive adhesive layer, this problem can be solved by coating the metal oxide particles with a polymer. The technique proposed by the Patent Document 3 could have a potential to improve the adherence property of the pressure-sensitive adhesive layer to some extent. However, it cannot solve most of the remaining problems pointed out in connection with the Patent Document 2. In particular, the technique described in the Patent Document 3 is based on coating the metal oxide particles with a particular polymer. Thus, the resulting pressure-sensitive adhesive becomes more costly than that in the Patent Document 2.